Telephone lines, when used to transmit a data signal between a transmitter and a receiver, introduce various disturbances to the transmitted signal. For example, the telephone lines may produce various kinds of phase error such as phase jitter, phase offset, and frequency offset. Phase jitter is rapid variations in the difference between the phase of the modulation carrier of the transmitter and the phase of the demodulation carrier of the receiver. Phase offset is an absolute time differential between the phase of the transmitter and the phase of the receiver. Frequency offset is the shifting of the entire spectrum of the transmitted signal. With respect to time, frequency offset has a generally linear phase characteristic. Although telephone lines introduce disturbances other than phase error, the present invention is primarily concerned with phase error.
Common Assignee's copending application Ser. No. 548,567 filed Feb. 10, 1975, now U.S. Pat. No. 3,971,996 and entitled PHASE LOCK LOOP discloses a receiver adapted to receive a transmitted signal from telephone lines. This copending application is a continuation of application Ser. No. 376,464 filed July 5, 1973, now abandoned, which in turn is a continuation-in-part of application Ser. No. 324,657 filed on Jan. 18, 1973, now abandoned. In the receiver of the copending application, the transmitted signal is demodulated and passed through low-pass filters to an equalizer which corrects the delay and attenuation distortion of the signal to provide an equalized signal. The phase error of the equalized signal is corrected in a phase correcting network to provide a phase corrected signal and the data in the phase corrected signal is detected by detectors. An error calculator responds to the phase corrected and detected signals to provide a phase error signal which is representative of phase error. The phase error signal is fed back to the phase correction network where appropriate adjustments are made to compensate for phase error.
The system of the copending application performs very satisfactorily. However, the error calculator does allow noise to be introduced into the phase error signal from one channel even when the signal in that channel does not represent any data or information.
A phase error equation which eliminates noise in the phase error signal from a channel when that channel contains no data or information is disclosed as equation 17 in U.S. Pat. No. 3,669,511 which issued to D. M. Motley, et al. However, this equation is relatively difficult for the error calculator to implement in that full accuracy multiplications are required and a storage capacity to hold large numbers, such as 10-bit numbers, is required.